1. Field of the Invention
Aspects of the present invention relate to an electrode assembly and a lithium secondary battery having the same, and more particularly, to a lithium secondary battery that is excellent in battery safety and battery performance.
2. Description of the Related Art
As portable electronic products that are small in size and light in weight are rapidly developed, it is desirable that the batteries used as power sources for driving the electronic products be small and have a high capacity. Specifically, a lithium secondary battery has an operating voltage of 3.6V or more, which is three times higher than that of a nickel cadmium battery or a nickel metal hydride battery, which are commonly used as power sources of portable electronic products. Furthermore, a lithium secondary battery has a high energy density per unit weight. Due to these reasons, lithium secondary batteries are being rapidly developed.
A lithium secondary battery generates electric energy by an oxidizing or reducing reaction when lithium ions are intercalated/deintercalated in positive/negative electrodes. A lithium secondary battery is manufactured by using materials that are capable of reversely intercalating/deintercalating lithium ions as active materials of the positive/negative electrodes and by charging an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.
Typically, a lithium secondary battery comprises an electrode assembly, a can and a cap assembly. The electrode assembly is formed of a negative electrode plate, a positive electrode plate and a separator between the negative electrode plate and the positive electrode plate, which are rolled in a predetermined shape, such as, for example, a jelly-roll shape. The can receives the electrode assembly and an electrolyte and the cap assembly is assembled on the cap.
The positive electrode plate of the electrode assembly is electrically connected to the cap assembly through a positive electrode lead, and the negative electrode plate of the electrode assembly is electrically connected to the can through a negative electrode lead.
The separator of the lithium secondary battery functions to prevent a short circuit by separating the positive electrode from the negative electrode and maintains high ion conductivity by storing the electrolyte required for a battery reaction. Specifically, in a lithium secondary battery, it is desirable to prevent movement of a material that obstructs the battery reaction and to secure the safety of the battery when an abnormality occurs. The separator typically uses a material such as a polyolefin based micro-porous high molecular layer such as polypropylene, polyethylene or the like, or a multi-layer thereof. In a conventional separator, since a porous layer is in the form of a sheet or film, pores of the porous layer may become blocked or the separator may contract by the generation of heat caused by an internal short circuit or over-charging. Therefore, when the sheet-type separator contracts and shrinks by the generation of heat inside the battery, the positive electrode and the negative electrode may contact each other directly where the separator and contracted or shrunk, and a fire, bursting or explosion may result.
Further, in a conventional film-type separator, when heat is generated by a short circuit, polypropylene or polyethylene resin softens to block pores, so that stability is secured by the shutdown function of interrupting the movement of lithium ions, that is, the flow of a current. However, the conventional film-type separator has a weak structure with respect to an internal short circuit. For example, in a nail test (to perforate through a battery using a nail) which is a substitution evaluation modeling an internal short circuit, since the heat generated upon an internal short circuit locally exceeds several hundreds ° C., depending on test conditions, a deformation of the porous layer occurs by the softening or loss of the resin. Then, when the nail perforates through the positive electrode and the negative electrode, abnormal over-heating occurs. Therefore, the shutdown effect of the resin is not an absolutely reliable safeguard against an internal short circuit.
Further, in the film-type separator, lithium dendrites result from over-charging. Since the separator is in the form of a film, a gap may occur between the negative electrode and the film and thus, lithium ions that do not enter inside the negative electrode accumulate on the surface of the negative electrode, that is, in the gap between the negative electrode and the film, and are deposited in the form of lithium metal dendrites. The deposited lithium dendrite pose a risk of perforating the film-type separator. If this happens, the positive electrode may contact the negative electrode or simultaneously, the lithium metal may undergo a side reaction with the electrolyte. Then, the battery may catch fire or explode by the heat and gas generated from the side reaction.
Moreover, when the film-type separator becomes misaligned due to vibration or falling, the separator cannot perform its intended function of separating the positive electrode from the negative electrode. As a result, the positive electrode contacts the negative electrode, causing a short circuit. Consequently, the battery cannot function. Moreover, when assembling a battery, mis-winding may occur, resulting in an increased rate of poor products and causing a problem of manufacturing stability. Moreover, since the film melts at a temperature of 100° C. or more, the film-type separator cannot be used at a high temperature.
To overcome the aforementioned problems of the film-type separator, research has been actively conducted to develop a ceramic separator that includes a porous layer formed by combining the ceramic material, such as silica (SiO2), alumina (Al2O3), zirconium oxide (ZrO2) or titanium oxide (TiO2), and a binder.
However, even when a ceramic material such as silica (SiO2), alumina (Al2O3), zirconium oxide (ZrO2) or titanium oxide (TiO2) is used as a separator, there is still a risk of an internal short circuit or combustion of the negative electrode active material.